The invention relates to a method for recycling exhaust gas of a multi-cylinder reciprocating internal combustion engine operated with an exhaust gas turbocharger, which for each cylinder possesses at least one inlet valve in an inlet duct connected with a blowing air manifold and at least one outlet valve in an outlet duct connected with an exhaust gas manifold and furthermore an exhaust gas recycling duct between the exhaust gas manifold and the blowing air manifold.
The invention is concerned with the following problem. It is prior art to reduce NOx emission of internal combustion engines by returning their exhaust gas to the induction side. In this case the exhaust gas is taken from an exhaust duct and returned to the induction system of the respective internal combustion engine. For optimum efficiency it is furthermore necessary to cool the recycled exhaust gas. However in the case of blown internal combustion engines and more particularly those with cooling of the blowing air to prevent fouling up the compressor and the blowing air cooler by residues of the exhaust gas, the exhaust gas is preferably tapped upstream from the turine, cooled and returned to the induction system at some point downstream from the blowing air cooler. In the characteristics of an internal combustion engine there are however many ranges, in which the mean exhaust gas counter pressure upstream from the turbine is greater than the mean blowing pressure downstream from the blowing air cooler. This means that in this operational range there will be, in the absence of special measures, a flow of the blowing air into the exhaust gas duct and not, as desired, of the exhaust gas into the induction system. Various measures are known for preventing the establishment of a flow in the wrong direction and also to ensure that an amount of exhaust gas, which is sufficient as regards the desired reduction of emission of NOx, may flow against the existing pressure gradient back to the starting point. One known means for this is the use of special-purpose check valves, so-called EGR flutter valves, in the exhaust gas recycling duct. In this case advantage is taken of pressure peaks occurring in the exhaust gas duct in order to open the EGR flutter valve and to cause the exhaust gas to flow to the induction side. When the pressure in the exhaust gas duct drops below the pressure of the blowing air, the EGR flutter valve, which is now closed, will prevent this resulting in a reversal of the direction of flow. This exhaust gas recycling by means of EGR flutter valves does however have certain disadvantages. The greater the efficiency of the turbocharger, the greater the mean pressure difference between the blowing air pressure and the counter pressure of the exhaust gas and the smaller the exhaust gas recycling rate, which can be attained. This means that improvements in fuel consumption obtainable by optimum designs of the turbocharger can not be attained, because then no optimized exhaust gas recycling rates can be produced. Furthermore, EGR flutter valves are subject to a high thermal load due to the recycled exhaust gas, this entailing an extremely high expenditure on design in order to be sure of getting the necessarily long service life and reliability for such EGR flutter valves. A disadvantage is furthermore that following disintegration of an EGR flutter valve, which are necessarily lacking in robustness, under the high dynamic loading, fragments of the valves will be induced by the internal combustion engine, something which then constitutes a substantial risk of damage to the engine. Besides the relatively high costs of development, the relatively expensive and complex manufacture of such EGR flutter valves is to be noted as a further disadvantage. A further disadvantage is that the exhaust gas must be cooled upstream from any flutter valve in order to not impair its service life, for which reason for each exhaust gas path with its separate exhaust gas recycling duct, its own EGR cooler, flutter valve, a shut off member is required upstream from the turbine in order to attain the desired exhaust gas recycling rates.
To round off the prior art attention is also to be paid to the MTZ Motortechnische Zeitschrift 60 (1999) 4 pages 240, 242. In section 3.3 there is a mention of a donor cylinder principle. This involves the employment of a cylinder solely for exhaust gas recycling and returning the exhaust gas ejected from this cylinder via an exhaust gas return duct with an EGR cooler directly to the blowing air manifold duct. This design is misdirected for a number or reasons and is furthermore not in accordance with practical requirements either.
One object of the invention is hence to provide a method for exhaust gas recycling for an internal combustion engine of the type initially mentioned, which may be performed using simple means and deals with the problems which have so far occurred in connection with EGR flutter valves. As regards designs, which deal with the above mentioned disadvantages and problems, it is to be borne in mind that the exhaust gas recycling means must be able to be turned off, for example when using the engine as a brake and however also when accelerating from low revs, it is absolutely necessary, for the sake of keeping down particle emission, to prevent exhaust gas from getting into the induction system. It is consequently necessary to take measures to see that return flow of the exhaust gas, for instance while using the engine as a brake, that exhaust gas return flow is reliably prevented.
In accordance with the characterizing part of claim 1 this object is to be achieved because recycling of exhaust gas is only permitted during certain operational phases of the internal combustion engine and during such exhaust gas recycling phases only the exhaust gas, which is ejected from one cylinder of a row of cylinders, is completely or partially recycled at a setting of the exhaust gas recycling rate via the exhaust gas return duct to the blowing air manifold duct, this exhaust gas return or recycling being however prevented outside i. e. between such exhaust gas recycling phases and the exhaust gas ejected from the cylinder or cylinders, just like the exhaust gas from the other cylinders, being completely recycled to the exhaust gas turbocharger via the exhaust gas manifold duct.
Advantageous embodiments and details of the design in accordance with the invention are recited in the dependent claims.
One principle of the method in accordance with the invention is that in the exhaust gas return or recycling phases the expulsion work of the piston of a cylinder of a row of cylinders of the internal combustion engine is directly employed for the recycling of exhaust gas. The method of the invention therefore entirely makes do without the so far required, expensive and sensitive and furthermore unreliable EGR flutter valves. Dependent on whether the exhaust gas outlet of the reciprocating internal combustion engine has one or two valves and how many cylinders the internal combustion engine has, it is merely necessary to provide at least one control member, which is under the control of a control device, with which during exhaust gas recycling phases only the exhaust gas expelled from one cylinder of a row of cylinders is returned completely or partially at a set exhaust gas recycling rate via the exhaust gas recycling duct to the induction system. In certain cases it is possible, in the simplest conceivable manner, to employ a design with one control member, which is only to be set in the positions EGR-on or EGR-off. In other cases, in which the control member may also be shifted into intermediate positions for the purpose of setting the recycling rate, the range of regulation may be so large that an exhaust gas recycling rate will be set which is optimized in accordance with requirement and is exactly adapted to the respective load state of the internal combustion engine.
In what follows the invention will be described in more detail with reference to the drawings, in which various types of multi-cylinder reciprocating internal combustion engines will be seen together with examples of details with which the method of the invention may be performed.